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People naturally swing their arms back and forth while running.
Prior research suggested that swinging the arms helps keep the
body
balanced during running by counteracting the motion of the
swinging legs. However, it was unclear whether swinging the arms
during running cost or saved people energy, and the few studies
exploring the question yielded contradictory results.

"One can imagine that not swinging the arms — that is, holding
them in the most relaxed manner possible — would be less
expensive metabolically speaking, since we wouldn't have to use
the muscles to hold the arms bent at the elbow and to swing the
arms back and forth," said lead study author Christopher
Arellano, a biomechanist at Brown University.

In the new study, the scientists looked at
people who ran often, recreationally and even competitively.
"Being in Boulder, Colorado, there is never a problem of
recruiting runners to be in our studies," Arellano told Live
Science.

The researchers first asked 13 runners to run normally on a
treadmill as they measured the rates at which they consumed
oxygen and exhaled carbon dioxide. They next asked the volunteers
to run without swinging their arms in three different ways — by
lightly clasping their hands behind their back, crossing the arms
across their chest, and holding their hands on the top of their
head. [ 5
Running Mistakes You Didn’t Know You Make ]

The scientists found that swinging the arms reduced energy costs
by 3 percent compared with holding the hands behind the back, 9
percent compared with holding the arms across the chest, and 13
percent compared with holding the hands on top of the head.

The investigators explained that even though
arm swinging costs energy, not swinging the arms during
running would cost even more energy. Arm swinging reduces torso
motion, and the torso accounts for 40 to 50 percent of the body's
weight, whereas both arms account for only about 10 percent of
the body's mass.

However, the researchers cautioned that people seeking to
burn more calories should not do so by trying to keep from
swinging their arms during running.

"When you constrain arm swing, the spine twists and rotates in
compensation," study co-author Rodger Kram, a biomechanist
at the University of Colorado at Boulder, told Live Science.
"Over the long term, that could be very hard on the spine."

"If you want to burn a little more calories, run a little
longer," Arellano said.

Arellano and Kram detailed their findings online July 16 in The
Journal of Experimental Biology.